1. Field of the Invention
The present invention relates to radio frequency identification tags (RF tags), and in particular, to an RF communication system and method for locating RF tags utilizing relative phase information from multiple channels.
2. Description of the Related Art
Remote communication utilizing wireless equipment typically relies on radio frequency (RF) technology, which is employed in many industries. One application of RF technology is in locating, identifying, and tracking objects, such as animals, inventory, and vehicles.
RF identification (RFID) tag systems have been developed that facilitate monitoring of remote objects. As shown in FIG. 1, a basic RFID system 10 includes two components: an interrogator or reader 12, and a transponder (commonly called an RF tag) 14. The interrogator 12 and RF tag 14 include respective antennas 16, 18. In operation, the interrogator 12 transmits through its antenna 16 a radio frequency interrogation signal 20 to the antenna 18 of the RF tag 14. In response to receiving the interrogation signal 20, the RF tag 14 produces an amplitude-modulated response signal 22 that is transmitted back to the interrogator 12 through the tag antenna 18 by a process known as backscatter.
The conventional RF tag 14 includes an amplitude modulator 24 with a switch 26, such as a MOS transistor, connected between the tag antenna 18 and ground. When the RF tag 14 is activated by the interrogation signal 20, a driver (not shown) creates a modulating signal 28 based on an information code, typically an identification code, stored in a non-volatile memory (not shown) of the RF tag 14. The modulating signal 28 is applied to a control terminal of the switch 26, which causes the switch 26 to alternately open and close. When the switch 26 is open, the tag antenna 18 reflects a portion of the interrogation signal 20 back to the interrogator 18 as a reflected portion 30 of the response signal 22. When the switch 26 is closed, the interrogation signal 20 travels through the switch 26 to ground, without being reflected, thereby creating a null portion 32 of the response signal 22. In other words, the interrogation signal 20 is amplitude-modulated to produce the response signal 22 by alternately reflecting and absorbing the interrogation signal 20 according to the modulating signal 28, which is characteristic of the stored information code. The RF tag 14 could also be modified so that the interrogation signal is reflected when the switch 26 is closed and absorbed when the switch 26 is open. Upon receiving the response signal 22, the interrogator 18 demodulates the response signal 22 to decode the information code represented by the response signal.
The substantial advantage of RFID systems is the non-contact, non-line-of-sight capability of the technology. The interrogator 12 emits the interrogation signal 20 with a range from one inch to one hundred feet or more, depending upon its power output and the radio frequency used. Tags can be read through a variety of substances such as smell, fog, ice, paint, dirt, and other visually and environmentally challenging conditions where bar codes or other optically-read technologies would be useless. RF tags can also be read at remarkable speeds, in most cases responding in less than one hundred milliseconds.
A typical RF tag system 10 will contain a number of RF tags 14 and the interrogator 12. There are three main categories of RF tags. These are beam-powered passive tags, battery-powered semi-passive tags, and active tags. Each operates in fundamentally different ways.
The beam-powered RF tag is often referred to as a passive device because it derives the energy needed for its operation from the interrogation signal beamed at it. The tag rectifies the field and changes the reflective characteristics of the tag itself, creating a change in reflectivity that is seen at the interrogator. A battery-powered semi-passive RFID tag operates in a similar fashion, modulating its RF cross section in order to reflect a delta to the interrogator to develop a communication link. Here, the battery is the source of the tag's operational power. Finally, in the active RF tag, a transmitter is used to create its own radio frequency energy powered by the battery.
The range of communication for such tags varies according to the transmission power of the interrogator 12 and the RF tag 14. Battery-powered tags operating at 2,450 MHz have traditionally been limited to less than ten meters in range. However, devices with sufficient power can reach up to 200 meters in range, depending on the frequency and environmental characteristics.
Although prior art communication systems can communicate with an RF tag that is within a certain distance of the interrogator (depending on the interrogator power), the interrogator cannot determine the location of the RF tag. For example, the prior art interrogator may be able to determine that there is an RF tag within 100 meters of the interrogator, but cannot determine whether it is 90 meters away or 45 meters away. In addition, the prior art interrogator cannot determine whether the RF tag is in front of, behind, or to either side of the interrogator. There are numerous applications for which such location information is important.